1. Field of the Invention
The present invention relates in general to active analog filters. In particular, the present invention relates to silicon active analog filtering buffer apparatus. More particularly, the present invention relates to a silicon active filtering buffer apparatus and the method of operation thereof employing waveform shaping over input signal to obtain filtered output without complex computations.
2. Technical Background
Conventional electronic filters are made of many inductive coils and capacitors arranged in ladder-like connections. This type of passive filters provides neither voltage nor current gain, and features low output impedance that makes them barely suitable for applications such as, for example, 100/10 MHz Mixed Fast Ethernet network applications. The bulky physical size of these passive filters also make them un-fit for applications such as PCMCIA peripheral cards which place severe requirements in component spacing.
Active filters fabricated as semiconductor devices are designed to replace, as well as enhance, filtering performance characteristics of passive filters. These active filters can be generally differentiated in two categories based on the technique of their implementation. One category is based on the analog filtering technique. The other is based on the DSP (digital signal processing) filtering technique in which all signals to be actively filtered are converted into binary data and then crunched. The processed digital data is then converted back into analog form by D/A converter.
Conventional active analog filters may be categorized into three general types. They are, namely, switched-capacitor filters, active operational amplifier (OP AMP) filters, and continuous-time filters.
Switched-capacitor filters employ switches for the emulation of resistors with large resistance values. With the assistance of active amplifiers, they may achieve the required filtering functionality. However, when the frequency of signals filtered increases, switched-capacitor filters suffer from disadvantageous effects such as signal feed-through and limitation over bandwidth gain.
An active OP AMP filter is basically a connection of resistor and capacitor network that employs the assistance of selected functional features of the OP AMP included to achieve the desired filtering functional characteristics, OP AMP's have, however, never been ideal in providing the necessary functionality in this category of active filters and have filtering bandwidth limitations, which render this type of active analog filters virtually useless in high frequency applications.
Continuous-time filters use the somewhat higher performance transconductor approach to emulate inductors or large-value capacitors necessary for implementing active analog filtering. With proper interconnection, better performance can be achieved. However, non-ideal effects of transconductors still place inadvertent effects on the performance of this type of active filter. Stability has been a major concern. Also, this type of filters suffer from small output voltage swings, making it difficult for delivering large signals in occasions of heavy loading. Power consumption is another concern. A transconductor and a capacitor are used to emulate one active component such as an inductor or a large-value capacitor. As a result, these current sources internal to a multiple of transconductors are required to maintain powered on for all time, and consuming considerable amount of power, even in idle mode of these active filters constructed utilizing transconductors.